The invention relates to methods for making various metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, titanium alloys and titanium aluminide alloys into engineering components by melting of the alloys in a vacuum or under a low partial pressure of inert gas and subsequent casting of the melt in molds machined from fine grained high density, high strength isotropic graphite molds under vacuum or under a low partial pressure of inert gas.
There is a need for improving the molding of various metallic alloys such as nickel, cobalt and iron based superalloys, nickel aluminides, stainless steel alloys, titanium alloys, titanium aluminide alloys, zirconium and zirconium base alloys. Metallic superalloys of highly alloyed nickel, cobalt, and/or iron based superalloys are difficult to fabricate by forging or machining. Moreover, conventional investment molds are used only one time for fabrication of castings of metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, titanium alloys and titanium aluminide alloys. This increases the cost of production.
The term superalloy is used in this application in conventional sense and describes the class of alloys developed for use in high temperature environments and typically having a yield strength in excess of 100 ksi at 1000 degrees F. Nickel base superalloys are widely used in gas turbine engines and have evolved greatly over the last 50 years. As used herein the term superalloy will mean a nickel base superalloy containing a substantial amount of the gamma prime (Ni3 Al) strengthening phase, preferably from about 30 to about 50 volume percent of the gamma prime phase. Representative of such class of alloys include the nickel base superalloys, many of which contain aluminum in an amount of at least about 5 weight % as well as one or more of other alloying elements, such as titanium, chromium, tungsten, tantalum, etc. and which are strengthened by solution heat treatment. Such nickel base superalloys are described in U.S. Pat. No. 4,209,348 to Duhl et al. and U.S. Pat. No. 4,719,080, both of which are incorporated herein by reference. Other nickel base superalloys are known to those skilled in the art and are described in the book entitled xe2x80x9cSuperalloys IIxe2x80x9d Sims et al., published by John Wiley and Sons, 1987, incorporated herein by reference.
Other references, incorporated herein by reference and related to superalloys and their processing, are cited below:
xe2x80x9cInvestment-cast superalloys challenge wrought materialsxe2x80x9d from Advanced Materials and Process, No. 4, pp. 107-108 (1990). xe2x80x9cSolidification Processingxe2x80x9d, editors B. J. Clark and M. Gardner, pp. 154-157 and 172-174, McGraw-Hill (1974).
xe2x80x9cPhase Transformations in Metals and Alloysxe2x80x9d, Van Nostrand Reinhold, D. A. Porter, p. 234 (1981).
Nazmy et al., The Effect of Advanced Fine Grain Casting Technology on the Static and Cyclic Properties of IN713LC. Conf: High Temperature Materials for Power Engineering 1990, pp. 1397-1404, Kluwer Academic Publishers (1990).
Bouse and Behrendt, Mechanical Properties of Microcast-X Alloy 718 Fine Grain Investment Castings. Conf: Superalloy 718: Metallurgy and Applications 1989, Publ:TMS, pp. 319-328 (1989).
Abstract of U.S.S.R. Inventor""s Certificate 1306641 (Published Apr. 30, 1987).
WPI Accession No. 85-090592/85 and Abstract of JP 60-40644 (KAWASAKI) (Published Mar. 4, 1985).
WPI Accession No. 814-06485D/81 and Abstract of JP 55-149747 (SOGO) (Published Nov. 21, 1980).
Fang, J: Yu, B Conference: High Temperature Alloys for Gas Turbines, 1982, Liege, Belgium, Oct. 4-6, 1982, Publ: D. Reidel Publishing Co., P.O. Box 17, 3300 AA Dordrecht, The Netherlands, pp. 987-997 (1982).
Processing techniques for superalloys have also evolved and many of the newer processes are quite costly.
U.S. Pat. No. 3,519,503 incorporated herein by reference describes an isothermal forging process for producing complex superalloy shapes. This process is currently widely used, and as currently practiced requires that the starting material be produced by powder metallurgy techniques. The reliance on powder metallurgy techniques makes this process expensive.
U.S. Pat. No. 4,574,015 incorporated herein by reference deals with a method for improving the forgeability of superalloys by producing overaged microstructures in such alloys. The xcex3xe2x80x2 (gamma prime) phase particle size is greatly increased over that which would normally be observed.
U.S. Pat. No. 4,579,602 deals with a superalloy forging sequence that involves an overage heat treatment.
U.S. Pat. No. 4,769,087 describes another forging sequence for superalloys.
U.S. Pat. No. 4,612,062 describes a forging sequence for producing a fine grained article from a nickel base superalloy.
U.S. Pat. No. 4,453,985 describes an isothermal forging process, which produces a fine grain product.
U.S. Pat. No. 2,977,222 describes a class of superalloys similar to those to which the invention process has particular applicability.
Titanium based alloys are also valuable for high performance uses. The major use of titanium castings is in the aerospace, chemical and energy industries. The aerospace applications generally require high performance cast parts, while the chemical and energy industries primarily use large castings where corrosion resistance is a major consideration in design and material choice.
The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material for many applications. Titanium alloys are used for static and rotating gas turbine engine components. Some of the most critical and highly stressed civilian and military airframe parts are made of these alloys.
The use of titanium has expanded in recent years from applications in food processing plants, from oil refinery heat exchangers to marine components and medical prostheses. However, the high cost of titanium alloy components may limit their use. The relatively high cost is often fabricating costs, and, usually most importantly, the metal removal costs incurred in obtaining the desired end-shape. As a result, in recent years a substantial effort has been focused on the development of net shape or near-net shape technologies such as powder metallurgy (PM), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used net shape technology. Titanium castings present certain advantages. The microstructure of as-cast titanium is desirable for many mechanical properties. It has good creep resistance, fatigue crack growth resistance, fracture resistance and tensile strength.
The casting of titanium and titanium alloys presents a special problem due to the high reactivity of the material in the molten state. This requires special melting, mold-making practices, and equipment to prevent alloy contamination.
The titanium casting industry is still in an early stage of development. Because of highly reactive characteristics of titanium with ceramic materials, expensive mold materials (yttrium, throe and zircon) are used to make investment molds for titanium castings. The titanium castings develop a contaminated surface layer due to reaction with hot ceramic mold and molten titanium. This surface layer needs to be removed by some expensive chemical milling in acidic solutions containing hydrofluoric acid). Strict EPA regulations have to be followed to pursue chemical milling.
For example, U.S. Pat. No. 5,630,465 to Feagin, incorporated herein by reference, discloses ceramic shell molds made from yttria slurries, for casting reactive metals. This patent is incorporated herein by reference.
The use of graphite in investment molds has been described in U.S. Pat. Nos. 3,241,200; 3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones and all incorporated herein by reference. U.S. Pat. Nos. 3,257,692 to Operhall; U.S. Pat. No. 3,485,288 to Zusman et al.; and U.S. Pat. No. 3,389,743 to Morozov et al. disclose carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders termed xe2x80x9cstuccosxe2x80x9d and are incorporated herein by reference.
U.S. Pat. No. 4,627,945 to Winkelbauer et al., incorporated herein by reference, discloses injection molding refractory shroud tubes made from alumina and from 1 to 30 weight percent calcined fluidized bed coke, as well as other ingredients. The ""945 patent also discloses that it is known to make isostatically-pressed refractory shroud tubes from a mixture of alumina and from 15 to 30 weight percent flake graphite, as well as other ingredients.
It is another object of the present invention to cast nickel, cobalt and iron base superalloys in isotropic fine grained graphite molds.
It is another object of the present invention to cast nickel aluminide alloys in isotropic fine grained graphite molds.
It is another object of the present invention to cast stainless steels in isotropic fine grained graphite molds.
It is another object of the present invention to cast titanium and titanium alloys in isotropic fine grained graphite molds.
It is another object of the present invention to cast titanium aluminides in isotropic fine grained graphite molds.
It is another objective of the present invention to cast zirconium and zirconium alloys in isotropic fine grained graphite molds.
It is another object of the present invention to provide isotropic graphite molds.
These and other objects of the present invention will be apparent from the following description.
This invention relates to a process for making various metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, titanium alloys and titanium aluminide alloys as engineering components by vacuum induction melting of the alloys and subsequent casting of the melt in graphite molds under vacuum. More particularly, this invention relates to the use of high density ultrafine grained isotropic graphite molds, the graphite of very high purity (containing negligible trace elements) being made via the isostatic pressing route. High density ( greater than 1.77 gm/cc), small porosity ( less than 13%), high flexural strength ( greater than 7,000 psi), high compressive strength ( greater than 9,000 psi) and fine grains ( less than 10 micron) are some of the characteristics of isostatically pressed graphite that render it suitable for use as molds for casting superalloys. The other important properties of the graphite material are high thermal shock, wear and chemical resistance, and minimum wetting by liquid metal. The extruded graphites which have lower density ( less than 1.72 gm/cc), lower flexural strength ( less than 3,000 psi), high porosity ( greater than 20%), lower compressive strength ( less than 8,000 psi) and coarse grains ( greater than 200 microns) have been found to be less suitable as molds for casting iron, nickel and cobalt base superalloys.
The present invention has a number of advantages:
(1) Use of ultrafine grained isotropic graphite molds to fabricate superalloy castings improves quality and achieves superior mechanical properties compared to castings made by a conventional investment casting process.
(2) The molds can be used repeatedly many times thereby reducing significantly the cost of fabrication of castings compared to traditional process.
(3) Near net shape parts can be cast, eliminating subsequent operating steps such as machining.
(4) The castings can be made in molds held at room or low temperatures resulting in finer grain structures and improved mechanical properties.